The invention relates to a process for exhaustion and treatment of gases from a sintering plant.
Making fine ores lumpy for use in blast furnaces is called sintering or agglomeration. In a conventional sintering plant a sinter mixture, i.e. a fine ore mixture and a fuel, is placed on a sintering belt, i.e. a conveyor belt designed as a grating. The sintering belt with the mixture, also called a sintering bed, passes over a large number of suction boxes. Air is drawn through the sintering bed via these suction boxes. An ignition furnace located above the first suction box ignites the sinter mix. The combustion spreads from the top downwards through the sintering bed during passage over the other suction boxes. During combustion the admixed fuel produces a temperature which is just sufficient to soften the fine ore at its surface, so that the fine ore mixture agglomerates to form a sinter cake. The combustion gases produced during sintering are exhausted with the combustion air via the suction boxes. The sintering plants are usually equipped in such a way that the suction boxes are connected via an electrostatic filter (in some cases also a fabric filter) to a fan, which generates the required negative pressure under the sintering belt to draw the required combustion air through the sintering bed. The cleaned gas mixture is then discharged by the fan into the atmosphere via a chimney stack.
Many of the sintering plants currently in operation exhibit major environmental problems. The gas mixture exhausted under the sintering belt has in fact a high dust and pollutant content. The gas mixture discharged from the chimney stack normally has dioxin contents of about 3-7 ng per m3 N.T.P. In addition, relatively high dust concentrations of more than 100 mg/m3 N.T.P. in older plants and less than 50 mg/m3 N.T.P. in more modern plants are achieved when electrostatic filters are used because of the unfavorable dust behavior in sintering plants.
Various methods have so far been used to solve the dioxin problem.
For example, a catalyzer which has already proved effective in refuse incinerators for dioxin separation has been used behind the electrostatic filter. In the refuse incinerators this catalyzer destroys the dioxin without residues. However, it has emerged during operation of such a catalyzer in a sintering plant that the dioxin destruction in the catalyzer is often disturbed and sometimes does not even take place at all. This is attributable in particular to the low gas temperatures (sometimes below 100xc2x0 C.).
In another process activated charcoal or hearth-furnace coke together with calcium hydroxide is injected into the waste gas flow behind the electrostatic filter. The activated charcoal bonds the dioxin, the calcium hydroxide is required to render the process inert. Because of the high reactivity of the activated charcoal/hearth-furnace coke there is otherwise a fire hazard. Behind the injection section there is a fabric filter, in which the injected substances contaminated with dioxin are separated again and the generally still quite high dust concentrations behind the electrostatic filter are further reduced significantly. Distribution of the activated charcoal/hearth-furnace coke proves to be problematical, because in the areas of overdosing there is a greater fire hazard and in areas of low concentration adequate dioxin separation is not achieved. Furthermore, a product heavily contaminated with dioxin, which must be further processed (e.g. by recycling to the sintering plant), is formed in the filter. A large proportion of the filter dust is returned to the injection point in order to use the activated charcoal/hearth-furnace coke repeatedly. Only a small partial flow is discharged and returned e.g. to the sintering plant.
According to a further process activated charcoal/hearth-furnace coke is injected directly into the electrostatic filter. However, there is considerable doubt as to whether the required dioxin separation can be achieved with this measure and whether the electrostatic filter allows larger quantities of dust to pass through as a result of the additional dust load. In addition substantially higher quantities of activated charcoal/hearth-furnace coke are required than in the preceding process. There is also the problem of disposal of the electrostatic filter material.
Consequently the task of the invention is to propose a process with which the waste gas problem of a sintering plant can be solved more effectively, easily and economically.
According to the invention this problem is solved by a process according to claim 1.
In a sintering plant the temperature in the sintering bed is relatively low from the ignition area to the center of the plant. The temperature rises clearly in the sintering bed only beyond the center of the plant. Hence it is possible to distinguish between a cold and a hot zone in the sintering plant. According to the invention a more effective, simpler and economical treatment of the waste gases of the sintering plant is achieved by exhausting and treating the gases from the cold zone and hot zone of the sintering plant as separate partial flows instead of exhausting and treating them as a total flow as in the past. Dioxin measurements on existing sintering plants have in fact revealed that only very small dioxin quantities are produced in the cold zone. Hence the partial flow exhausted separately from the cold zone is only very slightly contaminated by dioxin and does not require treatment for reduction of the dioxin content. Larger quantities of dioxin are released only in the waste gases from the hot zone and exhausted with the partial flow from the hot zone. If this second partial flow is to undergo treatment to reduce the dioxin content, it should be stated that as a result of separation of the partial flow from the cold zone the temperature of the partial flow from the hot zone does, of course, exceed the mixing temperature of the total flow, which has a positive effect, for example, on the efficiency of dioxin separation in the catalyzer. Hence it can be concluded that the waste gas problem of the sintering plant is generally simplified by the separate exhaustion of the gases from the cold and hot zone. A more specific and thus more effective gas treatment can take place, economic advantages likewise being achieved by the smaller gas quantities in the partial flows.
The partial flow from the cold zone of the sintering plant advantageously undergoes only dust removal treatment. As only very small dioxin quantities are present in the partial flow originating from the cold zone, it is unnecessary, for example, to subject this partial flow to treatment for reduction of the dioxin content.
Dust is preferably removed from the partial flow from the cold zone of the sintering plant in one or more electrostatic filters. The dust separation in electrostatic filters can be clearly improved by a higher H2O concentration in the cold zone, the lower waste gas temperature and a far smaller gas quantity.
The partial flow from the hot zone of the sintering plant advantageously first undergoes dust removal and is subsequently treated to reduce the dioxin content. Dust is preferably removed from the partial flow from the hot zone of the sintering plant in a fabric filter or cloth filter, which is particularly effective in the case of fine dusts in relatively dry gas mixtures.
The dioxin content is preferably reduced in a catalyzer. A catalyzer which permits extremely good dioxin separation in refuse incinerators, for example, can be used. Only the partial flow from the hot zone of the sintering plant, in which substantial dioxin quantities are released, is collected and fed to the catalyzer. As the partial flow treated in the catalyzer has an adequately high temperature, the dioxin is destroyed without residues in the catalyzer.
The partial flow from the hot zone of the sintering plant may be additionally heated in front of the catalyzer. The additional heating can advantageously take place e.g. by combustion of the CO gas present in this partial flow, preferably in a CO catalyzer. Consequently an even higher gas temperature is achieved, which further improves the dioxin separation in the catalyzer.
The partial flow from the hot zone of the sintering plant is preferably additionally subjected to treatment for reduction of the NOx content. This treatment for reduction of the NOx content advantageously comprises injection of NH3 into the partial flow from the hot zone of the sintering plant. At the temperatures of the partial flow from the hot zone NOx reacts well with NH3.
The partial flow from the hot zone preferably has a mixing temperature of over 200xc2x0 C. and the partial flow from the cold zone a mixing temperature of less than 100xc2x0 C. In fact, at temperatures of over 200xc2x0 C. the catalyzer should be capable of destroying the dioxin without residues and at temperatures of less than 100xc2x0 C. the dioxin content in the waste gas of the sintering plant should be negligible.
The dioxin content in the partial flow from the cold zone is preferably less than 0.5 ng/m3 N.T.P.
Under standard conditions (P=1 bar, T=273.15 K) the two partial flows are preferably of approximately the same size.